Non-volatile semiconductor memories, such as a split gate flash memory, typically use a stacked floating gate type field effect transistors, in which electrons are induced into a floating gate of a memory cell to be programmed by biasing a control gate and grounding a body region of a substrate on which the memory cell is formed.
An oxide-nitride-oxide (ONO) stack is used as either a charge storing layer, as in silicon-oxide-nitride-oxide-silicon (SONOS) transistor, or as an isolation layer between the floating gate and control gate, as in a split gate flash memory.
FIG. 1 is a partial cross-sectional view of an intermediate structure for a semiconductor device 100, such as a memory device, having a SONOS gate stack or structure 102 including a conventional ONO stack 104 formed over a surface 106 of a silicon substrate 108 according to a conventional method. In addition, the device 100 typically further includes one or more diffusion regions 110, such as source and drain regions, aligned to the gate stack and separated by a channel region 112. Briefly, the SONOS structure 102 includes a poly-silicon (poly) gate layer 114 formed upon and in contact with the ONO stack 104. The poly gate layer 114 is separated or electrically isolated from the substrate 108 by the ONO stack 104. The ONO stack 104 generally includes a lower oxide layer 116, a nitride or oxynitride layer 118 which serves as a charge storing or memory layer for the device 100, and a top, high-temperature oxide (HTO) layer 120 overlying the nitride or oxynitride layer.
One problem with conventional SONOS structures 102 and methods of forming the same is the poor data retention of the nitride or oxynitride layer 118 that limits the device 100 lifetime and/or its use in several applications due to leakage current through the layer.
Another problem with conventional SONOS structures 102 and methods of forming the same is the stoichiometry of the oxynitride layer 118 is neither uniform nor optimized across the thickness of the layer. In particular, the oxynitride layer 118 is conventionally formed or deposited in a single step using a single process gas mixture and fixed or constant processing conditions in an attempt to provide a homogeneous layer having a high nitrogen and high oxygen concentration across the thickness of the relatively thick layer. However, due to top and bottom effects this results in nitrogen, oxygen and silicon concentrations, which can vary throughout the conventional oxynitride layer 118. The top effect is caused by the order in which process gases are shut off following deposition. In particular, the silicon containing process gas, such as silane, is typically shut off first resulting in a top portion of the oxynitride layer 118 that is high in oxygen and/or nitride and low in silicon. Similarly, the bottom effect is caused by the order in which process gases are introduced to initiate deposition. In particular, the deposition of the oxynitride layer 118 typically follows an annealing step, resulting in a peak or relatively high concentration of ammonia (NH3) at the beginning of the deposition process and producing in a bottom portion of the oxynitride layer that is low in oxygen and silicon and high in nitrogen. The bottom effect is also due to surface nucleation phenomena in which that oxygen and silicon that is available in the initial process gas mixture preferentially reacts with silicon at the surface of the substrate and does not contribute to the formation of the oxynitride layer. Consequently, the charge storage characteristics, and in particular programming and erase speed and data retention of a memory device 100 made with the ONO stack 104, are adversely effected.
Accordingly, there is a need for a memory device having an ONO stack with an oxynitride layer as a memory layer that exhibits improved programming and erase speed and data retention. There is a further need for a method or process of forming an ONO stack having an oxynitride layer that exhibits improved oxynitride stoichiometry.